(1) Field of the Invention
The present invention relates to a process for the preparation of an aluminum borate whisker.
The aluminum borate whisker obtained according to the process of the present invention is excellent in mechanical strength, heat resistance, heat-insulating property, chemical resistance, electrically insulating property and neutron ray-absorbing property, and this whisker is valuable as a reinforcer for a thermoplastic resin, a thermosetting resin, cement, glass or metal.
(2) Description of the Related Art
Various processes have been tried for aluminum borate whiskers excellent in mechanical strength, chemical resistance and other properties, and these processes are roughly divided into the gas phase method and the liquid phase method using a flux.
As the gas phase method, the specification of U.S. Pat. No. 3,350,166 describes that by passing water vapor through gaseous aluminum fluoride and boron oxide at a temperature of 1000.degree. to 1400.degree. C., a whisker having a composition of 4Al.sub.2 0.sub.3.B.sub.2 0.sub.3 is obtained.
As the liquid phase method, the specification of U.S. Pat. No. 3,080,242 describes that by heating a mixture of aluminum hydroxide and boron oxide, a whisker having a composition of 3Al.sub.2 0.sub.3.B.sub.2 0.sub.3 or 9Al.sub.2 0.sub.3.B.sub.2 0.sub.3 is obtained. In this method, excessive boron oxide acts as the flux. Furthermore, from the report of Bureau of Mines, U.S.A., it is known that a whisker having a composition of 2.5Al.sub.2 0.sub.3.B.sub.2 0.sub.3 is obtained by heating a mixture of aluminum oxide, sodium tetraborate and lithium chloride at 1200.degree. C. and that a whisker having a composition of 4.8Al.sub.2 0.sub.3.B.sub.2 0.sub.3 is obtained by heating a mixture of aluminum oxide, sodium tetraborate and boron oxide at 1400.degree. C. [Bureau of Mines Report of Investigations, 6575 (1965),]. Also in these methods, the excessive sodium tetraborate/lithium chloride mixture or the excessive sodium tetraborate/boron oxide mixture acts as the flux, and in general, the boric acid component is incorporated in an amount of at least three equivalents and the excessive portion acts as the flux.
Non-whisker aluminum borate is obtained by pulverizing and mixing an aluminum source component and a boric anhydride source component and heating the mixture at a temperature of 700.degree. to 1200.degree. C. Since the aluminum source component is solid at the above-mentioned temperature, the reaction becomes heterogeneous and the aluminum source components exists in the reaction products. In order to grow a whisker in this reaction system, it is necessary to make the reaction mixture as homogeneous as possible and maintain a physical space for the growth of the whisker. For this purpose, boron oxide and an alkali metal borate are incorporated as a flux into the reaction mixture.
However, since the boron oxide and alkali metal borate have a high melt viscosity, the growing rate of the whisker is very low. Accordingly, the viscosity of the reaction mixture should be reduced by incorporating an alkali metal chloride or the like. According to this method, a large whisker of aluminum borate can be obtained, but the excessive boric anhydride source component tends to form a multi-component glass matrix together with the aluminum source component and the alkali component and the grown whisker is wrapped with this glass matrix. Therefore, the operation of eluting the glass matrix component becomes necessary and hence, a long time is required for isolation of the whisker. Furthermore, the method is defective in that since a part of the aluminum source component is used as the component of the glass matrix, the yield of the aluminum borate whisker is reduced.
Moreover, in the above-mentioned system, since the whole reaction mixture is completely melted, a reaction vessel having a shape of a crucible is necessary. If a platinum crucible stable to the molten salts is used, although the reaction product can be easily isolated after cooling, the cost of the apparatus increases and the process is not suitable for the industrial manufacturing method.
If ceramic crucible made of alumina, mullite or silica is used, the boron oxide component contained in the molten salt reacts very easily with the material of the crucible and the reactant is completely fusion-sticked to the inner surface of crucible. Accordingly, for the isolation of the whisker, it is necessary that the crucible with the content should be immersed in hydrochloric acid, followed by boiling. In this case, the crucible having the inner surface already deteriorated by the contact with the molten salts is further damaged, and it is impossible to use the crucible repeatedly.